In the past, subscriber terminal installations have usually been constructed on the basis of a synchronous mode of transfer in which the information to be interchanged occupies a channel having a data rate which is fixed, e.g. 64 kbit/s. Synchronous transfer mode is used in narrowband integrated services digital networks, N-ISDN.
Asynchronous transfer mode, ATM is expected to be used for implementing broadband integrated services networks BISDN which, in addition to providing continuity with N-ISDN services, opens the way to new services requiring data rates at multiple higher values. Asynchronous transfer mode has the particularity of being able to convey information at any data rate providing it remains below the maximum data rate of the transmission path used.
In asynchronous transfer mode, information is transported in the form of cells of fixed length. These cells comprise a header of fixed length and an information field which is also of fixed length. Information belonging to a particular call is identified from a virtual circuit number contained in the header of the cell. This mode of multiplexing information makes it possible to convey information at arbitrary rates without requiring any transmission frame structure other than the cell structure. It is assumed below, purely by way of example, that header length is four 8-bit bytes and that information field length is 32 bytes, giving a total cell length of 36 bytes. Naturally, the header and the information field could have lengths other than those mentioned above.
An object of the invention is to provide a subscriber installation based on asynchronous transfer mode and capable of providing both continuity for the services offered by synchronous terminals connected to an existing installation and also opening the way to new services at data rates that are not yet fully identified.
Another object of the invention is to provide a subscriber terminal installation that is capable of evolving to adapt to the needs of the user.